1. FIELD OF THE INVENTION
The present invention relates to a cyan-colored photosensitive toner which is sensitive to light having wavelengths in the range of 600 to 700 nm, and more particularly to a photosensitive toner which is mixed with magenta-colored and yellow-colored photosensitive toners for forming a color image by a single exposure and developing operation.
2. DESCRIPTION OF THE PRIOR ART
In recent years, a method of forming a color image using photosensitive toners by a single exposure and developing operation has been attracting attention. In such a method, three kinds of photosensitive toners are used which are respectively colored cyan, magenta, and yellow. The photosensitive toners are sensitive to the colors of light complementary to the respective toner colors; i.e., the cyan toner is sensitive to red light, the magenta toner to green light, and the yellow toner to blue light, each becoming conductive when exposed to its complementary light.
The following describes a color image forming process which uses a mixture of the three color photosensitive toners, i.e., a photosensitive toner mixture.
FIG. 2 is a schematic diagram showing the main part of an image forming apparatus using the photosensitive toners mentioned above. The apparatus comprises a mixing chamber 13 in which photosensitive toners 14 of cyan, magenta, and yellow are mixed and contained in prescribed proportions, a sleeve 13b and a stirrer 13c which are disposed inside the mixing chamber 13, a transparent conductive support drum 16 rotatably mounted adjacent to one side of the mixing chamber 13, a light source unit 30 disposed inside the support drum 16, and a transfer unit 18 to which a voltage of prescribed polarity is applied. The reference numeral 20 indicates a sheet of copy paper.
The mixing chamber 13 contains carrier 15. The three kinds of photosensitive toners 14 fed into the mixing chamber 13 and the carrier 15 already contained therein are stirred together by means of the stirrer 13c, thereby electrically charging the photosensitive toners 14 through friction. The thus charged photosensitive toners 14 are delivered together with the carrier 15 to the sleeve 13b. Then, the photosensitive toners 14 and the carrier 15 form a magnetic brush while being carried on the surface of the sleeve 13b by the rotation thereof. A bias voltage of prescribed polarity is applied to the sleeve 13b, so that an electric field is formed between the support drum 16 and the sleeve 13b.
The light source unit 30 disposed inside the support drum 16 comprises three light sources, i.e., a green light source 31, a blue light source 32, and a red light source 33, which are capable of outputting light of respective wavelengths to which the respective photosensitive toners 14 in the photosensitive toner mixture are sensitive, the light being emitted toward the surface of the support drum 16 from the respective color light sources 31, 32, and 33.
In the color-image forming process, the photosensitive toners 14 and the carrier 15 form a magnetic brush on the sleeve 13b as described above, and then the magnetic brush comes into contact with the support drum 16. At this time, because of the electric field formed between the support drum 16 and the sleeve 13b, the charged photosensitive toners 14 move from the sleeve 13b onto the support drum 16, so that a layer 17 of the photosensitive toner mixture is formed on the support drum 16. Using the light sources 31 to 33, slit exposure is performed on the photosensitive toner mixture layer 17. In the slit exposure, green light is projected through the support drum 16 onto the areas of the charged photosensitive toner mixture layer 17 that corresponds to the green parts of the image to be formed, causing the magenta toner in the photosensitive toner mixture layer 17 in those areas to become conductive and to lose its electric charge. On the other hand, the yellow and cyan toners do not become conductive and remain charged. Likewise, for the blue areas of the image, blue light is projected onto the charged photosensitive toner mixture corresponding to those areas, causing the yellow toner to become conductive and lose its electric charge, while the cyan and magenta toners remain charged. Furthermore, for the red areas of the image, red light is projected onto the charged photosensitive toner mixture corresponding to those areas, causing the cyan toner to become conductive and lose its electric charge, while the magenta and yellow toners remain charged.
Each photosensitive toner 14 that has thus lost its electric charge moves away from the support drum 16 and back to the sleeve 13b. That is, in the areas exposed to the green light, the yellow and cyan toners are held on the surface of the support drum 16. Likewise, in the areas exposed to the blue light, the cyan and magenta toners are held on the support drum 16, and furthermore, in the areas exposed to the red light, the magenta and yellow toners are held on the support drum 16.
With the rotation of the support drum 16, the charged photosensitive toners still held on the support drum 16 are then brought to the position facing the transfer unit 18, where the toners are transferred onto the copy paper 20 by means of the transfer unit 18 to which a voltage of prescribed polarity is applied. At this time, the yellow and cyan toners adhering to the areas on the support drum 16 exposed to the green light are transferred to the copy paper 20 to impart green color. Likewise, the cyan and magenta toners adhering to the areas on the support drum 16 exposed to the blue light are transferred to the copy paper 20 to give blue color, and furthermore, the magenta and yellow toners adhering to the areas on the support drum 16 exposed to the red light are transferred to the copy paper 20 to give a red color. Thus, a complete color image is formed on the copy paper 20.
In the photosensitive toners used in such a color image forming process, the respective ranges of wavelengths of light to which the three color photosensitive toners are sensitive must be separated from each other, so as to prevent mixing of colors in the produced image. For that purpose, three kinds of photosensitive toners, i.e., a yellow toner, a magenta toner, and a cyan toner, are used, which are respectively sensitive to different kinds of light having wavelengths in the ranges of 400 to 500 nm (blue light), 500 to 600 nm (green light), and 600 to 700 nm (red light). A conventional cyan-colored photosensitive toner which is sensitive to the light with wavelengths in the range of 600 to 700 nm usually contains a resinous binder, zinc oxide, and Bromophenol Blue as a sensitizing dye.
Although the above-mentioned photosensitive toner sensitized by Bromophenol Blue has high photosensitivity, it has a shortcoming in that its sensitivity range extends below the wavelength of 600 nm while showing hardly any sensitivity to the light with wavelengths in the range of 680 to 700 nm (this photosensitive toner exhibits the highest sensitivity to the light having wavelength of 620 nm). A red filter dye may be added to suppress the sensitivity of this photosensitive toner to the light of wavelengths below 600 nm. In this case, however, the electric charge-holding characteristic of the photosensitive toner deteriorates and the photosensitive toner mixture including such a toner takes on reddish color.